1. Field of Invention
This invention relates generally to communications, and more particularly, to compensating devices for optical filters.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
Currently this expansion of bandwidth is being accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions, i.e. channels, of the light spectrum. WDM is therefore the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. As this digital signal is passed across an optical network, it will be subject at various intervals to amplification and dispersion compensation. These functions may be implemented while passively dropping or adding individual channels. A channel may be dropped/added by a device comprising an optical circulator with coupled optical filters which operates as an add/drop multiplexer (mux). An add/drop mux may be used to implement a simple form of optical network.
There are already known various constructions of optical filters, among them those which utilize the Bragg effect for wavelength selective filtering. The fiber Bragg grating filter is constructed by imprinting at least one periodic grating in the core of an optical fiber. The imprint may be achieved by exposing the core through the cladding to the interference pattern of two ultraviolet beams at an angle to one another. This results in a grating oriented normal to the fiber axis so that it reflects only that light having a wavelength within a very narrow range, back along the fiber axis opposite to the original propagation direction so that such reflected light is guided in the core to the point at which the original light had been launched into the fiber core. This grating is substantially transparent to the light for wavelengths outside the aforementioned narrow band so that it does not affect the further propagation of such other light. The incorporated grating of this kind produces a narrow transmission notch and a commensurately narrow reflective peak in the spectrum of light propagating in the optical fiber in one or the other of its longitudinal directions. The frequency of light affected in this manner by the incorporated periodic grating is related to the periodicity of the grating.
The requirement for the use of these filters in optical communication is that they maintain their stability as to the center wavelength/frequency of the reflected light over time and over the broad range of operating temperatures experienced in the field. These filters however exhibit wavelength sensitivity to variations in temperatures. The frequency of light reflected by such optical filters varies directly with the temperature of the grating region. Thus as the temperature of the element increases the wavelength of the reflected light increases. This precludes the use of the Bragg grating as a frequency standard absent suitable packaging for compensating the device for temperature. The frequency also varies directly with a mechanical strain on the grating region; as the strain on the grating increases the wavelength of the reflected light increases. Thus a compensation method that employs the mechanical sensitivity of the fiber to strain may be used to counteract the thermal sensitivity of the fiber.
Apparatus for compensating optical filters have been disclosed in the prior art. U.S. Pat. No. 5,042,898 entitled xe2x80x9cIncorporated Bragg filter temperature compensated optical wave type devicexe2x80x9d issued on Aug. 27, 1991 discloses one such device. That device passively compensates for temperature variations with the announced effect of stabilizing the reflected wavelength of the filter. The device has a number of limitations including an inherently large form factor and a limited tuning ability.
What is needed is a compensating apparatus and method for optical filters with a reduced form factor and an increased tuning precision as compared with prior art devices.
A method and apparatus for compensating an optical filter is disclosed. The device substantially maintains the selected center wavelength in the optical filter across a range of operating temperatures. The optical filter may be used in a broad range of applications for one or more optical signals as a filter or a frequency reference. The device exhibits a low form factor and precise tuning capability.
In an embodiment of the invention an optical filter is disclosed for filtering a selected channel among a plurality of multiplexed channels of an optical communication system. The optical filter includes an optical fiber and an elongate housing. The optical fiber has a first side and a second side and a filter portion intermediate the first side and the second side, and the optical fiber for transmitting the multiplexed optical communication and the filter portion for filtering the selected channel. The elongate housing includes exposed end portions. The optical fiber is affixed at each exposed end portion. The filter portion is located between each of the exposed end portions. The exposed end portions exhibit between them a negative coefficient of thermal expansion sufficient to generate strains on the filter to substantially stabilize a filtered wavelength to substantially correspond with the selected channel during temperature variations.
In an alternate embodiment of the invention an optical filter is disclosed. The optical filter includes an optical fiber and an elongate housing. The optical fiber includes an intermediate filter portion. The optical fiber for transmitting the multiplexed optical communication and the filter portion for filtering the selected channel. The elongate housing is defined about a longitudinal axis and includes a first anchor pad and a second anchor pad. Opposing sides of said optic fiber adjacent the filter portion are affixed to a respective one of the first anchor pad and the second anchor pad within the elongate housing. At least one of the anchor pads is at least initially movable in a plane intersecting the longitudinal axis to vary a strain on the filter portion to tune a center wavelength of the selected channel.
In an alternate embodiment of the invention an optical filter for filtering an optical signal is disclosed. The optical filter includes an optical fiber and an elongate housing. The optical fiber with a first side and a second side and a filter portion intermediate the first side and the second side, and the optical fiber for filtering a selected wavelength of the optical signal. The elongate housing with opposing exposed end portions and said optical fiber affixed at each exposed end portion with the filter portion located between each of the exposed end portions, and the exposed end portions exhibit between them a negative coefficient of thermal expansion sufficient to generate strains on the filter to substantially stabilize the filtered wavelength during temperature variations.
In an alternate embodiment of the invention an optical filter for filtering an optical signal is disclosed. The optical filter includes an optical fiber and an elongate housing. The optical fiber includes an intermediate filter portion, and the optical fiber for transmitting the optical signal and the filter portion for filtering the optical signal. The elongate housing defined about a longitudinal axis and including a first anchor pad and a second anchor pad, and opposing sides of said optic fiber adjacent the filter portion affixed to a respective one of the first anchor pad and the second anchor pad within said elongate housing, and at least one of the anchor pads at least initially movable in a plane intersecting the longitudinal axis to vary a strain on the filter portion to tune a wavelength filtered by the filter portion.
In still another embodiment of the invention a method for compensating an optical filter to a selected channel within a plurality of multiplexed channels of an optical communication is disclosed. The method for compensating comprises the acts of:
tensioning an optical filter to a first strain level at which a wavelength of light filtered by the filter approximates a center wavelength of the selected channel;
affixing opposing sides of the optical filter to a first attachment pad and a second attachment pad displaced from one another along at least a z axis of an xyz Cartesian coordinate system; and
moving at least one of the anchor pads in a plane intersecting the z axis to increase and decrease a strain on the filter portion to tune the optical filter to the center wavelength of the selected channel.
In still another embodiment of the invention a method for compensating an optical filter to a selected wavelength is disclosed. The method for compensating comprises the acts of:
tensioning an optical filter to a first strain level at which a wavelength of light filtered by the filter approximates a wavelength of the selected wavelength;
affixing opposing sides of the optical filter to a first attachment pad and a second attachment pad separated from one another along at least a xe2x80x9czxe2x80x9d axis of an xyz axis Cartesian coordinate system; and
moving at least one of the anchor pads in a plane intersecting the z axis to increase and decrease a strain on the filter portion to tune the optical filter to the selected wavelength.